1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device.
2. Description of the Prior Art
A technique of forming a non-single crystal semiconductor layer on a glass substrate by reduced pressure CVD or plasma CVD followed by heating the substrate at about 600xc2x0 C. to thereby crystallize the layer into a polycrystal semiconductor layer has been well-known in the field of manufacture of semiconductor devices. Explaining the process of this thermal crystallization, first the temperature is raised from room temperature (i.e. initial stage), second the temperature is maintained at about 600xc2x0 C. (i.e. intermediate stage) for a few hours to several tens hours, and finally the temperature is lowered to room temperature (i.e. last stage). If a cheap glass substrate utilized for a large-sized liquid crystal display device and so on is subjected to the thermal crystallization process at about 600xc2x0 C., since a cheap glass substrate has its strain point at about 600xc2x0 C., the glass substrate shrinks (that is, the volume of the glass at the last stage becomes smaller than that at the initial stage) and thereby internal stress is caused in a semiconductor layer provided on the substrate. Further, photolithography pattern to be used in the subsequent process is deformed due to the shrink of the glass substrate, so that mask alignment in the further subsequent process becomes difficult to carry out. According to an experiment, interface state in the semiconductor layer formed on the substrate was high due to the internal stress, therefore electrical property of the semiconductor layer was bad. For the above reason, a technique for obtaining on a glass substrate a semiconductor layer of excellent electrical property has been required in the field of manufacture of semiconductor devices.
It is an object of the present invention to provide a method for manufacturing a semiconductor device of high performance.
It is another object of the present invention to provide a semiconductor device comprising a channel region of high field effect mobility.
It is a further object of the present invention to provide a semiconductor device of high performance.
In order to attain these and other objects, first a glass substrate is heated in a non-oxidizing atmosphere or an inactive gas atmosphere at a temperature not higher than strain point of the glass substrate, and then a non-single crystal semiconductor layer is formed on the substrate and the non-single crystal semiconductor layer is crystallized by heat in a non-oxidizing atmosphere or an inactive gas atmosphere. The volume of the glass substrate at a temperature, e.g., room temperature after the glass substrate heating process is smaller than that at the same temperature as the above, e.g., room temperature before the glass substrate heating process. That is, the glass substrate shrinks after being heated. By virtue of this glass substrate heating process, when the non-single crystal semiconductor layer formed on the glass substrate is crystallized by heat, the volume of the glass substrate is not decreased so much, that is, the shrink of the glass substrate is very little. Therefore, a crystallized semiconductor layer free from internal stress can be obtained by the crystallization process.
A non-single crystal semiconductor includes an amorphous semiconductor, a semi-amorphous semiconductor, a microcrystal semiconductor, and an imperfect polycrystal semiconductor. The microcrystal semiconductor is defined as a semiconductor in amorphous state in which crystal state is dispersed. On the other hand, the imperfect polycrystal semiconductor is defined as a semiconductor in polycrystal state in which crystal growth is imperfect, that is, crystals can be grown more.
Strain point of glass is defined as the temperature at which the viscosity of glass v is 4xc3x971014 poise (log(v)=14.5).